The structure of thermals in cumulus from airborne dual-Doppler radar observations.

摘要

Airborne dual-Doppler syntheses are used in combination with flight-level in situ data to explore the velocity fields in vertical and horizontal cross-sections of cumulus turrets. The multidimensional representations of the rising thermals led to an improved conceptual model of cumulus cloud growth.; The Wyoming Cloud Radar (WCR) on-board the University of Wyoming KingAir research aircraft is a 95 GHz Doppler radar capable of quasi-simultaneous multi-fixed-beam scanning. A technique was developed to analyze and merge data collected by pairs of WCR beams yielding two-dimensional wind field syntheses in horizontal or vertical planes. The gridding methodology can be customized to follow the scanned surface, with grid cell sizes between 30 and 45 m. The algorithm to solve the velocity inverse decomposition problem takes advantage of available a-priori and external information about the target and its environment. An estimate of the ambient winds is employed to evaluate the velocity component normal to the solution plane. The accuracy of the two-dimensional (2-D) velocity is on the order of 1-2 m s-1, and it is dependent on: the radar system design, the data collection process, and the weather target properties. A methodology to calculate each error component and to infer the maximum uncertainty in the retrieved 2-D velocity is discussed.; The clouds sampled in this study were cumulus congestus forming in cold, dry and nearly neutrally stable environments over land. Observations focused on newly-developed turrets and followed their evolution over periods of up to 15 min. The turrets ranged from 1 to 2 km in horizontal size. Measurable radar reflectivities covered vertical depths of up to 2 km. Cloud bases were at temperatures of 0 to -5°C, and cloud top temperatures were -20 to -30°C.; The cumuli formed and evolved through sequences of updraft pulses, or thermals, consisting of well-defined vortex-ring structures at the top, and trailing turbulent wake flows behind them. The rate of rise of the thermal is about half the maximum updraft speed. The reflectivity and velocity fields, in both horizontal and vertical transects, show that the kinematic patterns contribute to a spatial organization of the hydrometeors within the cloud volumes. Gradients and characteristic length scales are similar for the horizontal and vertical planes. (Abstract shortened by UMI.)